1. Field of the Invention
The present invention relates to an apparatus for removing bubbles in paint and a paint coating system having such an apparatus in which paint subjected to bubble-removing operations in the bubble removing apparatus is supplied to a paint coating device.
2. Description of the Related Art
There has been widely practiced the operations of cutting, perforating and bending of a metallic plate to form structural elements having desired shape and dimensions for various apparatuses and appliances.
Paint coating has often been applied to such structural elements in order to improve the outer appearance and give rust-preventing function. Heretofore, the paint coating to the metallic elements has been carried out after a metallic sheet has been worked by machining and so on so that they have a predetermined shape and dimensions. However, the metallic structural elements after they have been processed by machining generally had complicated steric shapes, and therefore, paint coating operations were troublesome; much amount of paint was needed, and manufacturing cost was pushed up.
Recently, there has been carried out a method that a metallic plate is previously coated at its entire surface with paint to obtain a pre-coated metal (PCM) and the PCM is processed by machining to manufacture the structural elements. In such method that the paint is applied uniformly to the entire surface, loss of the paint can be minimized and excellent paint coating is obtainable. Accordingly, manufacturing steps can be shortened in view of the entire process of the manufacturing of the structural elements in addition to saving energy and resources.
In manufacturing the PCM, a paint coating method such as a curtain flow coating method, a roll coating method is desirably used because the continuous formation of a coating layer can be obtained efficiently and quickly.
Paint generally has a high viscosity and often contains bubbles during manufacturing steps. When the paint in which bubbles are contained is used for painting, pin holes may be produced in a coated layer and a curtain-like coating layer may be broken when the curtain flow coating method is used, whereby stable, excellent paint coating can not be obtained. Therefore, it is necessary to remove bubbles in paint prior to the painting operation.
Such treatment of removing bubbles has been carried out in such a manner that bubbles in paint are expanded by reducing pressure in a sealed container containing the paint to thereby float the expanded bubbles on the free surface, so that the floating bubbles are broken.
However, the above-mentioned pressure reducing operations were insufficient to remove the bubbles in a short time because the viscosity of paint is fairly high. Therefore, it was difficult to supply a sufficient amount of paint, which has been subjected to the bubble-removing operation, to a paint coating device at a high feeding rate. In view of the above, there have been taken such measures that (1) paint is forcibly agitated, (2) a high speed vortical stream is resulted in paint, or (3) ultrasonic waves are applied to paint, in order to increase a rate of removing bubbles under a reduced pressure.
However, when the bubble removing operation using the above-mentioned acceleration method of (1) was taken, the rate of removing bubbles would not be sufficiently reduced. Further, a continuous treatment was difficult because it utilizes a batch process. In the case of the bubble removing operations using the method of (2), solid components such as pigment were apt to be separated due to the centrifugal force of the vortical stream. Further, a continuous treatment was difficult. In the bubble removing operations using the acceleration method of (3), there was easily caused the agglomeration of solid components such as pigment by the action of the ultrasonic waves. Further, the continuous treatment was difficult. In the above-mentioned methods, efficiency of the breaking of the floating bubbles was also low.
Accordingly, in an attempt to construct a paint coating system for manufacturing the PCM by combining the conventional bubble removing apparatus and a paint coating device, it was necessary to previously remove bubbles in paint by a bubble removing apparatus having a large capacity when paint coating was to be continuously carried out. In this case, the paint which was subjected to the bubble removing operations has to be stored in the apparatus. Accordingly, a large-sized bubble removing apparatus and a paint storing container were needed, and the size of a paint coating system could not be reduced. Further, when a conventional bubble removing apparatus was to be used, the quality of paint decreased depending on a bubble removing method, hence the quality of a coated layer was reduced.